1. Field of the Invention
The present invention relates to an image forming apparatus such as, a copying machine, a facsimile, a printer, etc., and more particularly to an image forming unit device including a belt-formed member and a belt device in which the belt-formed member drives accurately even when the belt-formed member temporarily separates from some of opposing members.
2. Discussion of the Background
As an image forming apparatus, a tandem multicolor image forming apparatus, that includes an intermediate transfer element supported by a plurality of supporting rollers and a plurality of photoconductive elements as opposing members (image bearing members) arranged side by side in a line opposite to the intermediate transfer element and contacting the intermediate transfer element is known (e.g. in Japanese Utility Model Laid-Open No. 59-192159 and Japanese Patent Laid-Open publication No. 8-160839). In the apparatus, visible images corresponding to respective colors formed on surfaces of respective photoconductive elements are transferred onto the intermediate transfer element one after another while being superimposed with each other (a primary transfer). The visible image thus formed on the intermediate transfer element is then transferred onto a transfer member a t one time (a secondary transfer) to form a multicolor image on the transfer member. In those multicolor image forming apparatuses, there are apparatuses configured such that a black and white image forming mode using a single photoconductive element and a multicolor image forming mode superimposing toner images of a plurality of colors with each other using a plurality of photoconductive elements are selectable.
FIG. 27 illustrates a fullcolor electrophotographic copying machine using liquid developer as an example of the above-described tandem multicolor image forming apparatus. In the apparatus, four drum-shaped photoconductive elements 501Y, 501M, 501C and 501B corresponding to respective colors of yellow Y, magenta M, cyan C and black BK are provided side by side in a line such that the axes of rotation of photoconductive elements are located in parallel and in the same plane. Around respective photoconductive elements 501Y, 501M, 501C and 501B rotating in a clockwise direction, charging devices, writing systems to form an electrostatic image by irradiation of beam light corresponding to respective colors, developing units with liquid developer for respective colors etc. (not shown) are provided respectively in an order of a liquid electrophotographic printing process. Further, an intermediate transfer belt 505 as an intermediate transfer member is supported by a tension roller 502, guide rollers 503 and 504 etc. so as to rotate in a counterclockwise direction. The intermediate transfer belt 505 is disposed so as to contact each primary transfer area of photoconductive elements 501Y, 501M, 501C and 501B. The intermediate transfer belt 505 is pressed by spanning rollers 506Y, 506M, 506C and 506B so that it windingly contacts respective photoconductive elements. An image on the intermediate transfer belt 505, which has been formed as a result of transferring images of respective colors (Y, M, C and BK) at the primary transfer areas of respective photoconductive elements 501Y, 501M, 501C and 501B superimposing one after another, is conveyed to a secondary transfer area where a portion of the intermediate transfer belt 505 spanned between guide rollers 503 and 504 contacts a secondary transfer roller 507. Then, the image is transferred onto a transfer sheet 508 at the secondary transfer area to form a multicolor image on the transfer sheet 508. Further, a cleaning device 509 is provided at a place where the intermediate transfer belt 505 is supported by the tension roller 502.
In the fullcolor electrophotographic copying machine with liquid developer, a color mode can be freely selected from among, for example, a single color mode and a multicolor mode with four colors (a full color mode), two colors or three colors. For example, when a single color mode (black color mode) is selected, a black color image is formed on the transfer sheet 508 using the photoconductive element 501B, electrophotographic copying process members and the intermediate transfer belt 505.
When a single color image forming operation is performed in the above-described electrophotographic copying machine having selectable single color and multicolor modes, inconveniences may be caused because the photoconductive elements which are not involved in the image forming operation are located in contact with or in close proximity to the intermediate transfer element.
For example, life times of the photoconductive elements may be decreased because the photoconductive elements are kept in contact with the intermediate transfer element even when the photoconductive elements are not involved in the image forming operation. In the apparatus illustrated in FIG. 27, even in the black color mode, photoconductive elements 501Y, 501M and 501C, which are not involved in the image forming operation, are kept in contact with the intermediate transfer belt 505 and are rubbed by it. Therefore the life times of these photoconductive elements may be decreased.
Further, when photoconductive elements which are not involved in the image forming operation are kept in contact with or in close proximity to the intermediate transfer element, developer remaining on the photoconductive elements may be flown by the intermediate transfer element and scattered inside the apparatus. Developer remaining on the photoconductive elements may also adhere to a surface of the intermediate transfer element, which results in unnecessary consumption of developer.
The above-described inconveniences such as the life times of opposing members, such as photoconductive elements being decreased due to unnecessary contact of a belt-formed member, such as the intermediate transfer element, with the opposing members are caused not only in the above-described exemplary construction where a plurality of photoconductive elements are located side by side in a line so as to oppose and contact the belt-formed intermediate transfer element, but also in a construction where a plurality of opposing members are disposed side by side in a line so as to oppose and contact a belt-formed member supported by a plurality of supporting rollers driven while being temporarily separated from part of the plurality of opposing members. The above-described inconveniences are also caused, for example, in a construction where a belt-formed photoconductive element drives while the belt-formed photoconductive element is temporarily separated from part of a plurality of developer bearing members as the plurality of opposing members, or in a construction where a belt-formed transfer sheet conveying member drives while the belt-formed transfer sheet conveying member is temporarily separated from part of a plurality of photoconductive elements as the plurality of opposing members. Further, the above-described scattering of developer and unnecessary consumption of the developer occur not only when the plurality of opposing members are located side by side in a line opposing and contacting the belt-formed member but also when the plurality of opposing members are located side by side in a line opposing the belt-formed member in close proximity.
For example, in Japanese Patent Laid-Open Publication No. 9-146383, an example of an image forming apparatus, configured such that a transfer sheet conveying belt partly moves to separate from three photoconductive elements out of four, is described.
The inventors discovered the following shortcoming as a result of a further study on a construction that enables the intermediate transfer element as the belt-formed member to separate from part of the plurality of photoconductive elements as the plurality of opposing members. When the intermediate transfer element is separated from part of the photoconductive elements that are not involved in the image forming operation, a tension of the intermediate transfer element may vary. For example, when the intermediate transfer element is configured to contact each of the photoconductive elements with a certain contacting angle in order to form a primary transfer nip of a required width between the intermediate transfer element and each photoconductive element, the tension of the intermediate transfer element may be decreased when the intermediate transfer element is separated from some of the photoconductive elements which are not in use. Further, when part of a plurality of supporting rollers pivot in order to separate the intermediate transfer element from part of the photoconductive elements which are not involved in the image forming operation, the tension of the intermediate transfer element may be decreased or increased depending on a position of a pivot.
When the intermediate transfer element is driven while the tension has varied, the intermediate transfer element may not be driven accurately. For example, when the intermediate transfer element is frictionally driven by rubber rollers, if the tension of the intermediate transfer element is decreased, the intermediate transfer element may not be accurately driven by the rubber rollers due to slides of the intermediate transfer element over the rubber rollers. Contrarily, if its tension is increased, a driving load imposed on the intermediate transfer element may become too excessive to drive the intermediate transfer element accurately. What is meant herein by saying that the intermediate transfer belt is driven accurately is to minimize a change in the speed of the intermediate transfer element.
The above-described inconvenience of inaccurate drive of a belt-formed intermediate transfer element due to a variation in the tension of the intermediate transfer element may be caused not only when a plurality of photoconductive elements are disposed side by side in a line opposing and contacting the belt-formed intermediate transfer element as described above, but also when a plurality of opposing members are arranged side by side in a line opposing and contacting or in close proximity to a belt-formed member supported by a plurality of supporting rollers frictionally driven while being temporarily separated from part of the plurality of opposing members. For example, the inconvenience may also be caused when a belt-formed photoconductive element is driven while being separated from part of a plurality of developer bearing members as a plurality of opposing members or when a belt-formed transfer sheet conveying member is driven while being temporarily separated from part of a plurality of photoconductive elements as a plurality of opposing members. Further, the inconvenience may also be caused not only when the plurality of opposing members are arranged side by side in a line so as to contact the belt-formed member but also when they are arranged side by side in a line so as to oppose the belt-formed member in close proximity.